We define the default action value for WSDL 1.1 faults [1] as
[target namespace][delimiter][port type name][delimiter][operation
name]Fault:[fault name]
IIRC part of our reasoning for using "Fault:" as part of the structure
rather than [delimiter] was that having a constant number of delimiters
in the pattern makes it easy to deconstruct the URI into its constituent
parts (all our actions have the same number of delimiters, though the
delimiter might also appear in the [target namespace]).
However, [delimiter] can be ":" which interacts with "Fault:" in a way
that negates that advantage.
Some options to consider:
1) replace "Fault:" with "[delimiter]Fault[delimiter]"
2) replace "Fault:" with "Fault*" where * is a character that is allowed
in URIs without escaping, allowed in XML without escaping, disallowed
from NCName, and not a [delimiter], i.e. one of "!$'()*+,;=".
[1] http://www.w3.org/TR/2005/WD-ws-addr-wsdl-20050413/#defactionwsdl11